The invention relates to a method and apparatus for manufacturing centrifuged, glass fibre-reinforced synthetic material pipes which involve introducing liquid, curable resin, which can contain a filler, together with glass fibres and additives for the curing process, possibly also with sand, into a rotating mould. Furthermore, the invention relates to centrifuged, glass fibre-reinforced pipes which comprise several layers of cut lengths of glass fibres, polyester resin comprising a filler and sand as well as internal and external cover layers, layers which are glass fibre-reinforced internally and externally comprising polyester resin and a filler as well as in the middle comprising a core part of sand, polyester resin, filler and glass fibres, alternatively centrifuged, glass fibre-reinforced pipes comprising several layers of cut lengths of glass fibres and polyester resin comprising a filler, with glass fibres and a core part of sand.
When manufacturing centrifuged, glass fibre-reinforced synthetic material pipes, in particular when filled polyester resins are used, the long term annular rigidity is extremely low. An aim of this invention is to improve the long term annular rigidity.
In the case of known methods of manufacturing sand-filled, centrifuged glass fibre-reinforced synthetic material pipes, the manufacturing time is extremely long. This is associated with the fact that the injection arm which is used to inject the material into the rotating mould is moved between different stations and that the curing time is long.
CH 684 326 describes a method where warm water is used to heat the centrifugal mould to 40xc2x0 C. to 50xc2x0 C. The mould is further heated as a result of the exothermal progression of the curing reaction. However, during the curing process the temperature in the outer part of the pipe remains equal to the temperature of the mould, i.e. approximately 50xc2x0 C. to 60xc2x0 C. but the temperature in the inner part rises to 80xc2x0 C. to 120xc2x0 C. As a result, the inner part of the pipe cures extremely well, but not so the part of the pipe where the heat is transmitted into the mould. Moreover, in the case of the known method the resin used for the outermost wall region of the pipe being manufactured has a gel time which amounts to 10% to 70% of the gel time of the resin used for the innermost wall region.
In practice it has proven difficult to maintain in the inner wall region the short gel time necessary for the compacting in the outermost layers not to be excessive. If excessive compacting occurs, then the glass content amounts to 70%. wt and as a consequence prevents a good curing process.
The subject of the invention is a method of manufacturing centrifuged, glass fibre-reinforced synthetic material pipes which involves introducing liquid, curable resin, which can contain a filler, together with glass fibres and additives for the curing processing, possibly also with sand, into a rotating mould. As the raw materials are introduced into the mould, the temperature of the mould amounts to between 40xc2x0 C. and 75xc2x0 C. and the heating capacity of the mould is so high that the raw materials do not cause the temperature of the mould to drop more than 10xc2x0 C. Warm water is used to heat the mould.
The method in accordance with the invention is performed in an extremely short time and it produces pipes of improved quality.
The capacity to manufacture the pipes is increased, as described below:
a) by using a curing temperature, a heat transmission technique and a system of curing which produces a rapid curing process without the formation of cracks;
b) by using apparatus where the injection machine has a fixed position and the centrifugal machines, are moved with the moulds, so that one machine after the other moves into the injection position.
The quality of the pipes is improved by virtue of the fact that a mixture of polyester resin comprising a high content of filler is used but in different layers in the pipe the mixture is diluted with pure resin so that the corresponding layers contain less filler.
The heating capacity of the mould is preferably so high that the raw materials do not cause the temperature of the mould to drop more than 10xc2x0 C.
By controlling the temperature of the mould, the mould heat is initially used to heat the raw materials introduced. During the curing process, heat is transmitted into the mould. As the pipe starts to contract, the majority of the reaction heat remains in the pipe.
Further, during the continuous manufacture of the pipes, the new pipe immediately after the old pipe, warm water is sprayed onto the mould, in the case of a preferred variant of the method just before the pipes are removed from the mould, so that the mould is at the same temperature which is necessary when manufacturing the new pipe.
Using these features, the curing temperature and the heat transmission technique render it possible for the pipes to cure rapidly without the formation of cracks and the finished pipes can be removed easily from the mould.
In accordance with one embodiment of the method according to the invention of manufacturing centrifuged glass fibre-reinforced synthetic material pipes liquid, curable resin, which can contain a filler, together with glass fibres and additives for the curing process, possibly also with sand, are introduced into a rotating mould which is at a temperature between 40xc2x0 C. and 75xc2x0 C. as the raw materials are being introduced. Once the exothermal reaction of the resin has commenced, warm water is sprayed onto the mould to maintain the temperature of the mould at least at 60xc2x0 C. until the pipe is removed.
This embodiment of the method renders it possible for the curing of the manufactured pipes to proceed in a uniform manner.
When using this embodiment of the method, the warm water can be in the temperature range of 55 to 80xc2x0 C.
Preferably, the temperature of the mould prior to the introduction of the raw materials is between 55xc2x0 C. and 75xc2x0 C.
In accordance with one advantageous embodiment of the method according to the invention, the temperature of the warm water and the period of time when it is sprayed onto the mould are selected such that the temperature of the mould remains at least at 70xc2x0 C. until the pipe has been removed.
Furthermore, an embodiment of the method of manufacturing centrifuged, glass fibre-reinforced synthetic material pipes is proposed wherein liquid, curable resin, which can contain a filler, together with glass fibres and additives for the curing process, possibly also with sand, is introduced into a rotating mould and the temperature of the mould as the raw materials are introduced is between 40xc2x0 C. and 75xc2x0 C. The resin used for the layers in the outer region of the pipe has a gel time which is equal to or greater than the gel time of the resin used for the inner region of the pipe. Resin comprising a filler content of at least 40% and warm water at a temperature of at least 60xc2x0 C. are used for the curing process.
This embodiment of the method renders it possible for the curing of the manufactured pipes to proceed in a uniform manner.
The gel time in the case of the method according to the invention is thus no shorter in the outer wall region than in the inner wall region. On the contrary, it can be expedient for it to be even longer in the outer wall region if the mould is heated to 50 to 60xc2x0 C.
Preferably, the glass fibre content in the inner region of the pipe is higher than in the outer region, the filler content being correspondingly higher in the outer region. In one advantageous embodiment of the method, all the layers in the inner region with the exception of the inner cover layer contain a filler. A filler is also present in all the layers where the glass fibres are aligned in the peripheral direction. The long-term annular rigidity of the pipes manufactured according to the invention is improved.
This characteristic can be improved by using long glass fibres, i.e., glass fibres of a length more than 50 mm, preferably more than 75 mm. The length of the glass fibres used in the inner region is preferably greater than the length of the glass fibres used in the outer region of the pipe. The use of long glass fibres renders it possible to manufacture pipes which have higher bursting pressures. However, the pipes also have a lower axial strength.
In the case of small pipe diameters, approx. up to 400 mm, the axial strength can be insufficient, especially in the case of pipes with low nominal pressures. It is therefore advantageous to use longer glass fibres only on the inner side and to use short fibres of a length from 25 to 50 mm on the outer side.
Advantageously, the polyester resin can be mixed into the filler-polyester resin mixture if the core layer is manufactured with sand in the middle of the pipe. In the case of a further exemplified embodiment of the invention, the polyester resin and/or styrene are mixed into the filler-polyester resin mixture if the reinforced layers are manufactured in the inner part of the pipe.